This invention relates to polishing slurry to be used for polishing the surface of a metallic as well as non-metallic object.
The surface of a metallic or a non-metallic object is usually polished with free abrading particles. Methods of polishing with free abrading particles are approximately divided into the tape-polishing which uses a polishing tape and the pad-polishing which uses a polishing pad.
The tape-polishing process is carried out by supplying slurry on the surface of the object and running a polishing tape while pressing it on the surface. The pad-polishing process is carried out by rotating a lapping plate with a polishing pad attached to its surface, supplying polishing slurry on the surface of the polishing pad and pressing the surface of the object on it. The polishing tape and the polishing pad used for these methods are produced by cutting a porous sheet of a woven or non-woven cloth or a foamed material into the shape of a tape or a pad. The slurry is obtained by dispersing abrading particles in a dispersant comprising a water-based aqueous solution containing water, glycols or alcohols. Hard particles of a material such as silica, alumina, diamond, cerium oxide, ceramics and glass are used as the abrading particles.
Besides the two polishing technologies mentioned above, a new polishing technology has recently been developed whereby no polishing pad is used and slurry is directly supplied to the surface of a lapping plate. The object is pressed onto the lapping plate and the surface of the object is polished as the lapping plate and the object are moved with respect to each other. According to a published method, micron-sized elastic particles and smaller hard particles are mixed together and a substrate surface is polished during a polishing process by the hard particles which are provisionally attached to the elastic particles. Since no pad is required by this method, the elastic particles each functioning as a pad, this polishing technology is sometimes referred to as the padless polishing. In this method, the hard particles become adhesively attached to the elastic particles electrostatically, by the van der Waales force or by a mechanical force, become separated and again become reattached. This condition of the hard particles separably attached to the elastic particles is sometimes referred to as “virtual attachment.”
For polishing an object such as a magnetic hard disk substrate, a semiconductor wafer or a liquid crystal glass substrate which must be prepared to be extremely smooth and flat, polishing slurry with very small abrading particles with average diameter of 0.001–0.5 μm are used in any of these polishing technologies. This is because it is believed that the number of particles per unit surface area that act on the object surface increases if the abrading particles for the slurry are made smaller and hence the object surface can be polished more accurately.
If the size of abrading particles to be mixed to the slurry is reduced, however, the stock removal rate of individual abrading particles becomes smaller and hence it takes longer for the polishing work and the throughput is adversely affected. The current problem, therefore, is that object surfaces cannot be polished quickly and accurately “simply” by reducing the size of the abrading particles in the slurry.